Complex oxide having n-type thermoelectric characteristics

ABSTRACT

The present invention provides a complex oxide having a composition represented by the formula La 1−x M x NiO 2.7−3.3  or (La 1−x M x ) 2 NiO 3.6−4.4  (wherein M is at least one element selected from the group consisting of Na, K, Li, Zn, Pb, Ba, Ca, Al, Nd, Bi and Y, and 0.01≦×≦0.8), the complex oxide having a negative Seebeck coefficient at 100° C. or higher. This complex oxide is a novel material which exhibits excellent properties as an n-type thermoelectric material.

This application is the U.S. National Phase under 35 U.S.C. §371 ofInternational Application PCT/JP03/02827, filed Mar. 11, 2003, whichclaims priority to Japanese Patent Application No. 2002-80258, filedMar. 22, 2002. The International Application was not published under PCTArticle 21(2) in English.

TECHNICAL FIELD

This invention relates to a complex oxide capable of achieving highperformance as an n-type thermoelectric material, and an n-typethermoelectric material using the complex oxide.

BACKGROUND ART

In Japan, only 30% of the primary energy supply is used as effectiveenergy, with about 70% being eventually lost to the atmosphere as heat.The heat generated by combustion in industrial plants,garbage-incineration facilities or the like is lost to the atmospherewithout conversion into other energy. In this way, we are wastefullydiscarding a vast amount of thermal energy, while acquiring only a smallamount of energy by combustion of fossil fuels or other means.

To increase the proportion of energy to be utilized, the thermal energycurrently lost to the atmosphere should be effectively used. For thispurpose, thermoelectric conversion, which directly converts thermalenergy to electrical energy, is an effective means. Thermoelectricconversion, which utilizes the Seebeck effect, is an energy conversionmethod for generating electricity by creating a difference intemperature between both ends of a thermoelectric material to produce adifference in electric potential. In this thermoelectric generation,electricity is generated simply by setting one end of a thermoelectricmaterial at a location heated to a high temperature by waste heat, andthe other end in the atmosphere (room temperature) and connectingconductive wires to both ends. This method entirely eliminates the needfor moving parts such as the motors or turbines generally required forpower generation. As a consequence, the method is economical and can becarried out without releasing gases due to combustion. Moreover, themethod can continuously generate electricity until the thermoelectricmaterial has deteriorated.

Therefore, thermoelectric generation is expected to play a role in theresolution of future energy problems. To realize thermoelectricgeneration, large amounts of a thermoelectric material that has a highthermoelectric conversion efficiency and excellent heat resistance,chemical durability, etc. will be required.

CoO₂-based layered oxides such as Ca₃CO₄O₉ have been reported assubstances that achieve excellent thermoelectric performance in the airat high temperatures. However, all such oxides have p-typethermoelectric properties, and are materials with a positive Seebeckcoefficient, i.e., materials in which the portion located at thehigh-temperature side has a low electric potential.

To produce a thermoelectric module using thermoelectric conversion,usually not only a p-type thermoelectric material but also an n-typethermoelectric material are needed. However, n-type thermoelectricmaterials that have excellent heat resistance, chemical durability,etc., and have a high thermoelectric conversion efficiency have not yetbeen found. Therefore, thermoelectric generation using waste heat hasnot yet become practical.

In such circumstances, the development of n-type thermoelectricmaterials that are composed of abundantly available elements and haveexcellent heat resistance, chemical durability, etc., and have a highthermoelectric conversion efficiency is greatly desired.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows X-ray diffraction patterns of the complex oxides obtainedin Examples 1 and 56.

FIG. 2 schematically shows the crystal structures of complex oxides 1and 2.

FIG. 3 is a schematic representation of a thermoelectric modulecomprising the complex oxide of the invention as a thermoelectricmaterial.

FIG. 4 is a graph showing the temperature dependency of the Seebeckcoefficient of the sintered complex oxide prepared in Example 1.

FIG. 5 is a graph showing the temperature dependency of the electricalresistivity of the sintered complex oxide prepared in Example 1.

DISCLOSURE OF THE INVENTION

The present invention has been made to solve the above problems. Aprincipal object of the invention is to provide a novel material thatachieves excellent performance as an n-type thermoelectric material.

The present inventors conducted extensive research to achieve the aboveobject and found that a complex oxide having a specific compositioncomprising La, Ni and O as essential elements and partially substitutedby specific elements has a negative Seebeck coefficient and a lowelectrical resistivity, thus possessing excellent properties as ann-type thermoelectric material. The invention has been accomplishedbased on this finding.

The present invention provides the following complex oxides and n-typethermoelectric materials using the complex oxides.

1. A complex oxide having a composition represented by the formulaLa_(1−x)M_(x)NiO_(2.7−3.3) wherein M is at least one element selectedfrom the group consisting of Na, K, Li, Zn, Pb, Ba, Ca, Al, Nd, Bi andY, and 0.01≦x≦0.8, the complex oxide having a negative Seebeckcoefficient at 100° C. or higher.

2. A complex oxide having a composition represented by the formulaLa_(1−x)M_(x)NiO_(2.7−3.3) wherein M is at least one element selectedfrom the group consisting of Na, K, Li, Zn, Pb, Ba, Ca, Al, Nd, Bi andY, and 0.01≦x≦0.8, the complex oxide having an electrical resistivity of10 mΩcm or less at 100° C. or higher.

3. A complex oxide having a composition represented by the formula(La_(1−x)M_(x))₂NiO_(3.6−4.4) wherein M is at least one element selectedfrom the group consisting of Na, K, Li, Zn, Pb, Ba, Ca, Al, Nd, Bi andY, and 0.01≦x≦0.8, the complex oxide having a negative Seebeckcoefficient at 100° C. or higher.

4. A complex oxide having a composition represented by the formula(La_(1−x)M_(x))₂NiO_(3.6−4.4) wherein M is at least one element selectedfrom the group consisting of Na, K, Li, Zn, Pb, Ba, Ca, Al, Nd, Bi andY, and 0.01≦x≦0.8, the complex oxide having an electrical resistivity of10 mΩcm or less at 100° C. or higher.

5. An n-type thermoelectric material comprising the complex oxide of anyone of items 1 to 4.

6. A thermoelectric module comprising the n-type thermoelectric materialof item 5.

The complex oxide of the invention is an oxide whose composition isrepresented by the formula La_(1−x)M_(x)NiO_(2.7−3.3) (hereinafterreferred to as “complex oxide 1”), or an oxide whose composition isrepresented by the formula (La_(1−x)M_(x))₂NiO_(3.6−4.4) (hereinafterreferred to as “complex oxide 2”). In complex oxides 1 and 2, M is atleast one element selected from the group consisting of Na, K, Li, Zn,Pb, Ba, Ca, Al, Nd, Bi and Y, and is a value of 0.01 or more and 0.8 orless.

Complex oxides 1 and 2 have a negative Seebeck coefficient and exhibitproperties as n-type thermoelectric materials in that when a differencein temperature is created between both ends of the oxide material, theelectric potential generated by the thermoelectromotive force is higherat the high-temperature side than at the low-temperature side. Morespecifically, complex oxides 1 and 2 have a negative Seebeck coefficientat 100° C. or higher of, for example, about −1 to about −20 μVK⁻¹.

Furthermore, complex oxides 1 and 2 have good electrical conductivityand low electrical resistivity, and more specifically, an electricalresistivity of 10 mΩcm or less at 100° C. or higher.

FIG. 1 shows an X-ray diffraction pattern of the complex oxide obtainedin Example 1 given below, i.e., one embodiment of complex oxide 1. FIG.1 also shows an X-ray diffraction pattern of the complex oxide obtainedin Example 56 given below, i.e., one embodiment of complex oxide 2.

The X-ray diffraction patterns, although showing the presence of smallamounts of impurities, clearly indicate that complex oxide 1 has aperovskite-type crystal structure and complex oxide 2 has the so-calledlayered perovskite-type structure, thus being a perovskite-relatedmaterial.

FIG. 2 schematically shows the crystal structures of complex oxides 1and 2. As shown in FIG. 2, complex oxide 1 has a perovskite-type LaNiO₃structure in which the La sites are partially substituted by M andcomplex oxide 2 has a layered perovskite-type La₂NiO₄ structure in whichthe La sites are partially substituted by M.

Complex oxides 1 and 2 can be prepared by mixing the starting materialsin such a proportion so as to have the same metal component ratio as thedesired complex oxide, followed by sintering. More specifically, thestarting materials are mixed to have the same La/M/Ni metal componentratio as in the formula La_(1−x)M_(x)NiO_(2.7−3.3) or(La_(1−x)M_(x))₂NiO_(3.6−4.4) (wherein M and x are as defined above) andthe resulting mixture is sintered to provide the desired complex oxide.

The starting materials are not particularly limited insofar as theyproduce oxides when sintered. Examples of usable materials includemetals, oxides, compounds (such as carbonates), and the like. Examplesof usable sources of La are lanthanum oxide (La₂O₃), lanthanum carbonate(La₂(CO₃)₃), lanthanum nitrate (La(NO₃)₃), lanthanum chloride (LaCl₃),lanthanum hydroxide (La(OH)₃), lanthanum alkoxides (such asdimethoxylanthanum (La (OCH₃)₃), diethoxylanthanum (La(OC₂H₅)₃) anddipropoxylanthanum (La(OC₃H₇)₃), and the like. Examples of usablesources of Ni are nickel oxide (NiO), nickel nitrate (Ni (NO₃)₂), nickelchloride (NiCl₂), nickel hydroxide (Ni(OH)₂), nickel alkoxides (such asdimethoxynickel (Ni(OCH₃)₂), diethoxynickel (Ni(OC₂H₅)₂) anddipropoxynickel (Ni(OC₃H₇)₂), and the like. Similarly, examples ofusable sources of other elements are oxides, chlorides, carbonates,nitrates, hydroxides, alkoxides and the like. Compounds containing twoor more constituent elements of the complex oxide of the invention arealso usable.

The sintering temperature and sintering time are not particularlylimited insofar as the desired complex oxide can be produced under suchconditions. For example, the sintering may be performed at about 850° C.to about 1000° C. for about 20 to about 40 hours. When carbonates,organic compounds or the like are used as starting materials, thestarting materials are preferably decomposed by calcination prior tosintering, and then sintered to give the desired complex oxide. Forexample, when carbonates are used as starting materials, they may becalcined at about 600° C. to about 800° C. for about 10 hours, and thensintered under the above-mentioned conditions.

Sintering means are not particularly limited and any desired means suchas electric furnaces and gas furnaces may be used. Usually, sinteringmay be conducted in an oxidizing atmosphere such as in an oxygen stream,or in the air. When the starting materials contain a sufficient amountof oxygen, sintering in an inert atmosphere, for example, is alsopossible.

The amount of oxygen in a complex oxide to be produced can be controlledby adjusting the partial pressure of oxygen during sintering, sinteringtemperature, sintering time, etc. The higher the partial pressure ofoxygen is, the higher the oxygen ratio in the above formulae can be.

The thus obtained complex oxides 1 and 2 of the invention have anegative Seebeck coefficient and a low electrical resistivity, i.e., anelectrical resistivity of 10 mΩcm or less at 100° C. or higher, so thatthe oxides exhibit excellent thermoelectric conversion capabilities asn-type thermoelectric materials. Furthermore, the complex oxides havegood heat resistance and chemical durability and are composed oflow-toxicity elements and therefore highly practical as thermoelectricconversion materials.

The complex oxides 1 and 2 of the invention with the above-mentionedproperties can be effectively used as n-type thermoelectric materials inair at high temperatures.

FIG. 3 is a schematic representation of a thermoelectric module producedusing a thermoelectric material comprising a complex oxide of theinvention as its n-type thermoelectric elements. The thermoelectricmodule has a similar structure to conventional thermoelectric modulesand comprises a high-temperature side substrate, a low-temperature sidesubstrate, p-type thermoelectric materials, n-type thermoelectricmaterials, electrodes, and conductive wires. In such a module, thecomplex oxide of the invention is used as an n-type thermoelectricmaterial.

As described above, the complex oxides of the invention have a negativeSeebeck coefficient and a low electrical resistivity and are alsoexcellent in terms of heat resistance, chemical durability, etc.

The complex oxides of the invention with such properties can beeffectively utilized as n-type thermoelectric materials in air at hightemperatures, whereas such use is impossible with conventionalintermetallic compounds. Accordingly, by incorporating the complexoxides of the invention as n-type thermoelectric elements into athermoelectric module, it becomes possible to effectively utilizethermal energy conventionally lost to the atmosphere.

BEST MODE FOR CARRYING OUT THE INVENTION

Examples are given below to illustrate the invention in further detail.

EXAMPLE 1

Using lanthanum carbonate (La₂(CO₃)₃) as a source of La, nickel oxide(NiO) as a source of Ni, and potassium carbonate (K₂CO₃) as a source ofK, these starting materials were well mixed in a La/Ni/K ratio (elementratio) of 0.8:1.0:0.2. The mixture was placed into an alumina crucibleand calcined in the air using an electric furnace at 600° C. for 10hours to decompose the carbonates. The calcinate was milled and moldedby pressing, followed by sintering in an oxygen stream at 920° C. for 40hours to prepare a complex oxide.

The complex oxide thus obtained had a composition represented by theformula La_(0.8)K_(0.2)NiO_(3.2).

FIG. 4 is a graph showing the temperature dependency of the Seebeckcoefficient (S) of the obtained oxide over the temperature range of 100°C. to 700° C. It is apparent from FIG. 4 that the complex oxide has anegative Seebeck coefficient at 100° C. or higher, thus being confirmedto be an n-type thermoelectric material in which the high-temperatureside has a high electric potential.

Like Example 1, in all the Examples described below, the Seebeckcoefficient at 100° C. or higher was negative and showed a tendency tobecome more negative with a rise in temperature.

FIG. 5 is a graph showing the temperature dependency of the electricalresistivity of the complex oxide obtained in Example 1. FIG. 5demonstrates that the complex oxide shows a low electrical resistivity,i.e., an electrical resistivity of about 10 mΩcm or less over thetemperature range of 100° C. to 700° C.

EXAMPLES 2-110

Starting materials were mixed in the La/M/Ni ratios (element ratios)shown in Tables 1 to 4, and the same procedure as in Example 1 was thenrepeated to provide complex oxides.

The starting materials were those used in Example 1 and the followingmaterials: sodium carbonate (Na₂CO₃) was used as a source of Na, lithiumcarbonate (Li₂CO₃) as a source of Li, zinc oxide (ZnO) as a source ofZn, lead oxide (PbO) as a source of Pb, barium carbonate (BaCO₃) as asource of Ba, calcium carbonate (CaCO₃) as a source of Ca, aluminiumoxide (Al₂O₃) as a source of Al, neodymium oxide (Nd₂O₃) as a source ofNd, bisumuth oxide (Bi₂O₃) as a source of Bi, and yttrium oxide (Y₂O₃)as a source of Y.

The sintering temperature was selected from the range of 850° C. to 920°C. according to the desired complex oxide.

The complex oxides obtained in Examples 1 to 55 had a perovskite-typeLaNiO₃ structure in which the La sites were partially substituted by M,whereas those obtained in Examples 56 to 110 had a layeredperovskite-type La₂NiO₄ structure in which the La sites were partiallysubstituted by M.

Tables 1 to 4 below show the element ratios of the obtained complexoxides, their Seebeck coefficients at 700° C., and their electricalresistivity at 700° C.

TABLE 1 Formula: La_(1−x)M_(x)NiO_(y) Seebeck Electrical coefficientresistivity at 700° C. at 700° C. No. M La:M:Ni:O (μVK⁻¹) (mΩcm) 1 K0.8:0.2:1:3.2 −10 8 2 K 0.95:0.05:1:3.3 −8 5 3 K 0.9:0.1:1:3.2 −5 7 4 K0.5:0.5:1:3.1 −4 4 5 K 0.2:0.8:1:3.3 −3 4 6 Na 0.99:0.01:1:3.2 −7 7 7 Na0.95:0.05:1:3 −7 5 8 Na 0.9:0.1:1:2.9 −3 8 9 Na 0.5:0.5:1:3.0 −12 4 10Na 0.2:0.8:1:2.8 −5 6 11 Li 0.99:0.01:1:3.1 −18 8 12 Li 0.95:0.05:1:3.2−10 9 13 Li 0.9:0.1:1:2.8 −5 7 14 Li 0.5:0.5:1:2.7 −8 4 15 Li0.2:0.8:1:3.1 −3 7 16 Zn 0.99:0.01:1:2.8 −7 8 17 Zn 0.95:0.05:1:3.2 −8 518 Zn 0.9:0.1:1:2.7 −5 6 19 Zn 0.5:0.5:1:3.3 −8 4 20 Zn 0.2:0.8:1:3.2 −35 21 Pb 0.99:0.01:1:3.0 −10 8 22 Pb 0.95:0.05:1:2.9 −9 5 23 Pb0.9:0.1:1:3.1 −5 3 24 Pb 0.5:0.5:1:3.0 −7 4 25 Pb 0.2:0.8:1:2.8 −2 9 26Ba 0.99:0.01:1:3.2 −11 8 27 Ba 0.95:0.05:1:3.3 −7 5 28 Ba 0.9:0.1:1:3.1−5 6

TABLE 2 Formula: La_(1−x)M_(x)NiO_(y) Seebeck Electrical coefficientresistivity at 700° C. at 700° C. No. M La:M:Ni:O (μVK⁻¹) (mΩcm) 29 Ba0.5:0.5:1:2.8 −4 4 30 Ba 0.2:0.8:1:2.9 −3 3 31 Ca 0.99:0.01:1:3.1 −12 832 Ca 0.95:0.05:1:3.0 −8 6 33 Ca 0.9:0.1:1:3.3 −6 7 34 Ca 0.5:0.5:1:3.2−4 4 35 Ca 0.2:0.8:1:2.8 −7 7 36 Al 0.99:0.01:1:3.2 −10 8 37 Al0.95:0.05:1:2.9 −8 5 38 Al 0.9:0.1:1:3.1 −8 7 39 Al 0.5:0.5:1:3.0 −6 440 Al 0.2:0.8:1:3.3 −5 6 41 Nd 0.99:0.01:1:2.9 −12 8 42 Nd0.95:0.05:1:2.9 −9 7 43 Nd 0.9:0.1:1:3.1 −5 6 44 Nd 0.5:0.5:1:2.8 −4 445 Nd 0.2:0.8:1:3.1 −3 4 46 Bi 0.99:0.01:1:3.2 −10 8 47 Bi0.95:0.05:1:3.0 −8 3 48 Bi 0.9:0.1:1:2.8 −7 7 49 Bi 0.5:0.5:1:2.9 −4 550 Bi 0.2:0.8:1:3.0 −4 4 51 Y 0.99:0.01:1:3.2 −10 9 52 Y 0.95:0.05:1:3.3−8 5 53 Y 0.9:0.1:1:3.2 −5 4 54 Y 0.5:0.5:1:3.0 −8 4 55 Y 0.2:0.8:1:2.8−3 2

TABLE 3 Formula: (La_(1−x)M_(x))₂NiO_(y) Seebeck Electrical coefficientresistivity at 700° C. at 700° C. No. M La:M:Ni:O (μVK⁻¹) (mΩcm) 56 Na1.98:0.02:1:3.7 −11 9 57 Na 1.9:0.1:1:3.9 −8 7 58 Na 1.8:0.2:1:3.8 −4 759 Na 1:1:1:3.8 −7 6 60 Na 0.4:1.6:1:4.0 −3 4 61 K 1.98:0.02:1:3.9 −9 862 K 1.9:0.1:1:4.1 −8 9 63 K 1.8:0.2:1:3.6 −6 7 64 K 1:1:1:3.7 −4 7 65 K0.4:1.6:1:4.2 −5 8 66 Li 1.98:0.02:1:4.4 −11 8 67 Li 1.9:0.1:1:3.8 −8 568 Li 1.8:0.2:1:3.7 −9 7 69 Li 1:1:1:3.8 −4 5 70 Li 0.4:1.6:1:4.1 −5 471 Zn 1.98:0.02:1:4.2 −10 8 72 Zn 1.9:0.1:1:4.0 −7 7 73 Zn 1.8:0.2:1:3.9−5 7 74 Zn 1:1:1:3.8 −4 4 75 Zn 0.4:1.6:1:4.1 −9 9 76 Pb 1.98:0.02:1:4.2−10 8 77 Pb 1.9:0.1:1:3.7 −11 7 78 Pb 1.8:0.2:1:3.9 −5 7 79 Pb 1:1:1:3.8−5 4 80 Pb 0.4:1.6:1:4.2 −3 4 81 Ba 1.98:0.02:1:4.3 −6 8 82 Ba1.9:0.1:1:4.2 −8 6 83 Ba 1.8:0.2:1:4.4 −12 7

TABLE 4 Formula: (La_(1−x)M_(x))₂NiO_(y) Seebeck Electrical coefficientresistivity at 700° C. at 700° C. No. M La:M:Ni:O (μVK⁻¹) (mΩcm) 84 Ba1:1:1:3.9 −4 4 85 Ba 0.4:1.6:1:3.8 −16 4 86 Ca 1.98:0.02:1:3.9 −10 8 87Ca 1.9:0.1:1:4.1 −3 9 88 Ca 1.8:0.2:1:4.2 −5 7 89 Ca 1:1:1:4.3 −7 4 90Ca 0.4:1.6:1:4.0 −3 8 91 Al 1.98:0.02:1:3.9 −10 8 92 Al 1.9:0.1:1:3.8 −65 93 Al 1.8:0.2:1:4.0 −5 7 94 Al 1:1:1:4.1 −4 6 95 Al 0.4:1.6:1:3.8 −4 496 Nd 1.98:0.02:1:4.0 −10 8 97 Nd 1.9:0.1:1:3.9 −12 7 98 Nd1.8:0.2:1:3.7 −5 7 99 Nd 1:1:1:4.2 −4 8 100 Nd 0.4:1.6:1:3.8 −4 4 101 Bi1.98:0.02:1:4.1 −13 8 102 Bi 1.9:0.1:1:4.0 −4 6 103 Bi 1.8:0.2:1:4.2 −57 104 Bi 1:1:1:3.9 −9 8 105 Bi 0.4:1.6:1:4.3 −3 4 106 Y 1.98:0.02:1:4.0−10 8 107 Y 1.9:0.1:1:4.1 −8 5 108 Y 1.8:0.2:1:3.9 −7 7 109 Y 1:1:1:4.0−4 4 110 Y 0.4:1.6:1:4.1 −5 9

1. A complex oxide having a composition represented by the formulaLa_(1−x)M_(x)NiO_(2.7−3.3) wherein M is at least one element selectedfrom the group consisting of Na, K, Li, Zn, Pb, Ba, Al, Nd, Bi and Y,and 0.01≦x≦0.8, the complex oxide having a negative Seebeck coefficientat 100° C. or higher.
 2. A complex oxide having a compositionrepresented by the formula La_(1−x)M_(x)NiO_(2.7−3.3) wherein M is atleast one element selected from the group consisting of Na, K, Li, Zn,Pb, Ba, Al, Nd, Bi and Y, and 0.01≦x≦0.8, the complex oxide having anelectrical resistivity of 10 mΩcm or less at 100° C. or higher.
 3. Acomplex oxide having a composition represented by the formula(La_(1−x)M_(x))₂NiO_(3.6−4.4) wherein M is at least one element selectedfrom the group consisting of Na, K, Li, Pb, Ca, Al, Nd, Bi and Y, and0.01≦x≦0.8, the complex oxide having a negative Seebeck coefficient at100° C. or higher.
 4. A complex oxide having a composition representedby the formula (La_(1−x)M_(x))₂NiO_(3.6−4.4) wherein M is at least oneelement selected from the group consisting of Na, K, Li, Pb, Ca, Al, Nd,Bi and Y, and 0.01≦x≦0.8, the complex oxide having an electricalresistivity of 10 mΩcm or less at 100° C. or higher.
 5. An n-typethermoelectric material comprising the complex oxide of any one ofclaims 1 to
 4. 6. A thermoelectric module comprising the n-typethermoelectric material of claim
 5. 7. A sintered complex oxide ofLa_(1−x)M_(x)NiO_(2.7−3.3) wherein M is at least one element selectedfrom the group consisting of Na, K, Li, Zn, Pb, Ba, Al, Nd, Bi and Y,and 0.01≦x≦0.8.
 8. The sintered complex oxide according to claim 7,which has a negative Seebeck coefficient at 100° C. or higher and/or anelectrical resistivity of 10 mΩcm or less at 100° C. or higher.
 9. Asintered complex oxide of (La_(1−x)M_(x))₂NiO_(3.6−4.4) wherein M is atleast one element selected from the group consisting of Na, K, Li, Zn,Pb, Ba, Ca, Al, Nd, Bi and Y, and 0.01≦x≦0.8.
 10. The sintered complexoxide according to claim 9, which has a negative Seebeck coefficient at100° C. or higher and/or an electrical resistivity of 10 mΩcm or less at100° C. or higher.
 11. A method of producing the sintered complex oxideof claim 9, comprising: mixing starting materials in a metal componentratio of (La_(1−x)M_(x))₂NiO_(3.6−4.4) wherein M is at least one elementselected from the group consisting of Na, K, Li, Zn, Pb, Ba, Ca, Al, Nd,Bi and Y, and 0.01≦x≦0.8; and sintering the mixture.
 12. The methodaccording to claim 11, wherein the sintering step is conducted at atemperature of about 850° C. to about 1,000° C.
 13. The method accordingto claim 11, further comprising subjecting the starting materials tocalcinations prior to the sintering step.
 14. A method of producing thesintered complex oxide of claim 7, comprising: mixing starting materialsin a metal component ratio of La_(1−x)M_(x)NiO_(2.7−3.3) wherein M is atleast one element selected from the group consisting of Na, Li, Zn, Pb,Ba, Al, Nd, Bi and Y, and 0.01≦x≦0.8; and sintering the mixture.
 15. Themethod according to claim 14, wherein the sintering step is conducted ata temperature of about 850° C. to about 1,000° C.
 16. The methodaccording to claim 14, further comprising subjecting the startingmaterials to calcinations prior to the sintering step.
 17. Athermoelectric module comprising: at least one p-type thermoelectricmaterial and at least one n-type thermoelectric material; the n-typethermoelectric material being a sintered body of a complex oxide havinga composition represented by the formula La_(1−x)M_(x)NiO_(2.7−3.3)wherein M is at least one element selected from the group consisting ofNa, K, Li, Zn, Pb, Ba, Ca, Al, Nd, Bi and Y, and 0.01≦x≦0.8; and thecomplex oxide having a negative Seebeck coefficient at 100° C. orhigher.
 18. A thermoelectric conversion method comprising: creating adifference in temperature between both ends of the thermoelectric moduleaccording to claim 17.